CN1608168B - Water-based viscoelastic fracturing fluid and hydrocarbon recovery method using same - Google Patents

Abstract

The present invention relates to a water-based viscoelastic fracturing fluid for recovering hydrocarbons. According to the present invention, the fracturing fluid contains a viscoelastic surfactant and a hydrophobically modified polymer, wherein the concentration of the hydrophobically modified polymer is between its approximately overlap concentration c ^* and its approximately entanglement concentration _ce .

Description

Water-based viscoelastic fracturing fluid and hydrocarbon recovery method using the fracturing fluid

technical field

The present invention relates to a water-based fracturing fluid for hydrocarbon recovery.

Background technique

Hydrocarbons, such as oil or natural gas, are obtained from hydrocarbon-bearing subterranean geological formations by drilling wells that provide partial flow pathways that allow the hydrocarbons to reach the surface. The hydrocarbons flow through flow channels connecting the reservoir in the formation to the wellbore.

However, blockage of the flow path may result in insufficient hydrocarbon production. In this case, different techniques can be applied to stimulate the production of hydrocarbons. Common among these techniques is the injection of specific fluids into the formation by drilling at sufficient pressure to create fractures in the formation rock. Flow channels can thus be created through which hydrocarbons can more easily flow into the well. The latter technique is known as fracturing or hydraulic fracturing, and the specific fluid used in this technique is known as a fracturing fluid.

Ideally, the fracturing fluid should have minimal pressure drop in the tubing within the well during placement of the fracturing fluid, and should have sufficient viscosity to carry the propping material used to prevent fracture closure. Additionally, the fracturing fluid should have a minimal leak rate to avoid migration of the fracturing fluid into the formation rock so that fractures can be significantly created and propped, and it degrades so as not to leave residues that could prevent precise hydrocarbon flow into the well. residual material.

Early fracturing fluids consisted of viscous or gel-like oils due to the belief that water-induced formation damage might not be as severe as originally thought. Said polymer gels include guar gum, guar gum derivatives or hydroxyethyl cellulose. In order to achieve sufficient fluid viscosity and thermal stability in high temperature reservoirs, linear polymer gels are partially cross-linked polymer gels such as those cross-linked by borate, zirconate or titanate ions replaced. But since it became clear that cross-linked polymer gel residues could impair the permeability of hydrocarbon-bearing formations, fluids with lower polymer content and foam fluids were added. Additionally, methods have been introduced to improve the cleaning of polymer-based fracturing fluids. These include advanced fracturing techniques.

Recently, polymer-free water-based fracturing fluids based on viscoelastic surfactants have been developed. The main advantages of viscoelastic surfactant fluids are ease of preparation, minimal formation damage, and high retained permeability in proppant. Viscoelastic surfactant fluids are disclosed in particular in the following patent documents: US-4,615,825, US-4,725,372, US-4,735,731, CA-1298697, US-5,551,516, US-5,964,295, US-5,979,555 and US-6,232,274. A well-known polymer-free water-based fracturing fluid containing a viscoelastic surfactant is the quaternary ammonium salt, N-callenyl-N,N-bis(2-hydroxyethyl)-N-methyl A mixture of ammonium chloride with isopropanol and brine, preferably comprising 3 wt% ammonium chloride and 4 wt% potassium chloride, this water-based fracturing fluid is commercially available from the Schlumberger group of companies under the trade name ClearFRAC ^™ . The viscoelastic surfactant molecules are present in sufficient concentration to aggregate into overlapping worm or rod micelles which impart the necessary viscosity to the fluid to carry the proppant during the fracturing process.

Under the flow conditions experienced during pumping and flow downhole, such viscoelastic surfactant-based fluids impart a relatively low frictional pressure drop, which is advantageous in applications involving the application of coiled tubing. In addition, viscoelastic surfactant-based fluids are "responsive" in the sense that they degrade to lower viscosity fluids by contacting and interacting with formation fluids, particularly hydrocarbons, during flow from the reservoir back to the wellbore.

On the other hand, the seepage rate of viscoelastic surfactant-based fracturing fluids may be high, which may limit its application in hydrocarbon-bearing formations where the formation rock permeability is very low. In addition, especially at high temperature conditions , the application of high concentrations of viscoelastic surfactants must meet operating regulations, which may increase the cost of fracturing fluids.

A patent entitled "Water-Dispersible Hydrophobic Thickening Agent" disclosed in US-4,432,881 and a patent entitled "Hydraulic Fracturing Process and Composition" disclosed in WO 87/01758 Compositions)" discloses fracturing fluids comprising nonionic surfactants and hydrophobically modified polymers. However, these documents do not mention viscoelastic surfactants or the responsiveness of said fluids to hydrocarbons. Hydrophobically modified polymers were added in varying concentrations along with other compounds, simply to increase the viscosity of the fluid. Likewise, a patent entitled "Method of Using a Foamed Fracturing Fluid" disclosed in US-5,566,760 discloses fracturing fluids comprising surfactants and hydrophobically modified polymers. In these fluids, surfactant molecules form an interface between gas bubbles and polymer molecules that form a polymer network similar to pure polymer fluids. There is still no mention of viscoelastic surfactants or the responsiveness of the fluid to hydrocarbons in this patent.

Contents of the invention

In view of the prior art described above, the present invention presents a need to solve the problem of water-based viscoelastic fracturing fluids for hydrocarbon recovery that are responsive to hydrocarbons and contain limited amounts of surfactants and/or or polymers, thereby reducing the costs involved in applying the fracturing fluid.

As a solution to the above problems, in a first aspect, the present invention relates to a water-based viscoelastic fracturing fluid for hydrocarbon recovery comprising a viscoelastic surfactant and a hydrophobically modified polymeric wherein the concentration of the hydrophobically modified polymer is advantageously between its approximate overlap concentration c ^* and its approximate entanglement concentration c _e .

In a second aspect, the present invention relates to a method of recovering hydrocarbons, the method comprising the steps of:

Provided is a water-based viscoelastic fracturing fluid comprising a viscoelastic surfactant and a hydrophobically modified polymer, wherein the concentration of the hydrophobically modified polymer is advantageously from about its overlap concentration c ^* to about its entanglement concentration c between _e ; and

The fracturing fluid is injected into formation rock, thereby fracturing the rock.

Hydrophobically modified polymers, especially the hydrophobic side chains of said polymers, interact with surfactant micelles. As a result, viscoelastic gel structures are formed at concentrations of viscoelastic surfactants lower than those typically employed with pure viscoelastic surfactant systems, thereby reducing the costs associated with applying the fluid, however due to the The fact that the polymer concentration is not sufficient to form an entangled gel network, the fluid remains responsive to hydrocarbons, and this polymer concentration is preferred.

Another important property affecting fluid efficiency and thus overall cost reduction is that the responsive mixtures of the present invention exhibit lower leakage rates relative to responsive fluids based on pure viscoelastic surfactants with the same rheological properties Behavior. For pure viscoelastic surfactant systems, the mixtures of the invention are expected to have a lower frictional pressure drop during pumping compared to pure polymer systems.

The viscoelastic surfactant is advantageously a cleavable viscoelastic surfactant and the hydrophobically modified polymer comprises cleavable hydrophobic cleavable side chains.

Description of drawings

The invention may be better understood on the basis of the following description of non-limiting and exemplary embodiments, with reference to the accompanying drawings, in which:

Figure 1 depicts the physical interactions that exist between hydrophobically modified polymers and rod-like surfactant micelles in fluids of the present invention;

Figure 2 compares the overlapping concentrations of cationic viscoelastic surfactants at 25, 40 and 60 °C;

Figure 3 compares the overlap and entanglement concentrations of hydrophobically modified polymers at 25, 40, 60, and 80 °C;

Figure 4 compares the viscosity of different fluids as a function of cationic viscoelastic surfactant concentration for different hydrophobically modified hydroxypropyl guar concentrations at a high shear rate of 100 s ^-1 ;

Figure 5 shows three bottles. It describes that the fluids of the present invention delay the formation of emulsions compared to the same fluid without the hydrophobically modified polymer component or without the cationic viscoelastic surfactant component;

Figure 6 compares the viscosity of cationic viscoelastic surfactant fluids containing different concentrations of hydrophobically modified hydroxypropyl guar as a function of time, and it allows the measurement of the break-up time of said fluids in case of contact with hydrocarbons and viscosity;

Figure 7 shows two bottles, which describe the need for hydrophobic side chains on the polymer backbone to create a stable interaction between the polymer network and the surfactant network, thus avoiding phase separation;

Figures 8 and 9 compare, as a function of surfactant concentration, the viscosity between a fluid comprising a hydrophobically modified polymer and a cationic viscoelastic surfactant and a corresponding fluid comprising an anionic viscoelastic surfactant;

Figure 10 compares the viscosity change of fluids comprising hydrophobically modified polymers and/or cationic viscoelastic surfactants as a function of temperature below 140°C at high shear rates of 100 s ^-1 ;

Figure 11 compares the concentration of hydrophobically modified polymers and/or zwitterionic viscoelastic surfactants as a function of the concentration of zwitterionic viscoelastic surfactants at high shear rates of 100 ^s Fluid viscosity change;

Figure 12 compares the viscosity change of fluids comprising zwitterionic viscoelastic surfactants and/or hydrophobically modified polymers as a function of temperature at a high shear rate of 100 s ^-1 ;

Figure 13 depicts the synthetic route of hydrophobically modified poly(ethylene-alternate-maleic anhydride);

Figure 14 compares the viscosities of four fluids comprising hydrophobically modified polymers and viscoelastic surfactants as a function of temperature;

Figure 15 describes the synthetic route of hydrophobically modified polyglucosamine;

Figure 16a compares the change in zero-shear viscosity of different fluids containing hydrophobically modified polyglucosamine and/or viscoelastic surfactants at 25°C as a function of the concentration of hydrophobically modified polyglucosamine ;

Figure 16b compares the change in zero-shear viscosity of different fluids containing hydrophobically modified polyglucosamine and/or viscoelastic surfactants at 60°C as a function of the concentration of hydrophobically modified polyglucosamine ;

Figure 17 compares the leakage volume over time for a viscoelastic fluid and a viscoelastic fluid comprising a hydrophobically modified polymer in a saline saturated core with a saline permeability of 7-9 mD;

Figure 18 compares the time-dependent leakage volume of viscoelastic fluid in brine-saturated or oil-saturated cores with a brine permeability of 7-9 mD;

Figure 19 compares the leakage volume over time for viscoelastic fluids comprising hydrophobically modified polymers in saline saturated cores or oil saturated cores with a saline permeability of 7-9 mD;

Figure 20 compares the effect of hydrophobically modified polymer concentration on the leakage rate of hydrophobically modified polymer/viscoelastic surfactant fluids;

Figure 21 compares the leakage volume over time for viscoelastic surfactant fluids with or without hydrophobically modified polymers for two ranges of permeability in oil saturated cores;

Figure 22 compares the leakage volume over time for viscoelastic fluids with and without hydrophobically modified polymers in oil saturated cores with and without fluid loss additives.

Detailed ways

The present invention relates to a water-based fluid for use in the recovery of hydrocarbons such as oil and gas. This water-based fluid is a fracturing fluid.

The fluids of the present invention comprise a viscoelastic surfactant and a hydrophobically modified polymer.

Unlike many other surfactants that form Newtonian solutions that are slightly more viscous than water at higher concentrations, the surfactants of the present invention are viscoelastic, capable of forming viscoelastic fluids at lower concentrations. This specific rheological behavior is mainly due to the type of surfactant aggregates present in the fluid. In low-viscosity fluids, surfactant molecules aggregate into spherical micelles, while in viscoelastic fluids, long micelles, which can be described as worm-like, thread-like, or rod-like glues, can exist and become entangled bundle.

The viscoelastic surfactants of the present invention are generally ionic. Depending on the charge of its head group, it can be cationic, anionic or zwitterionic. When the surfactant is cationic, it is associated with a negative counterion, which can be an inorganic anion such as sulfate, nitrate, perchlorate or a halogen such as Cl ^- , Br ^- or an aromatic organic anion such as salicylate , naphthalenesulfonate, p- and m-chlorobenzoate, 3,5 and 3,4 and 2,4-dichlorobenzoate, tert-butyl and ethyl phenolate, 2,6 and 2,5-di Chlorophenolate, 2,4,5-trichlorophenolate, 2,3,5,6-tetrachlorophenolate, p-methylcarbonate, m-chlorophenolate, 3,5,6-tri Chloropyridine, 4-amino-3,5,6-trichloropyridine, 2,4-dichlorophenoxyacetate. When the surfactant is anionic, it is linked to a positive and negative ion such as Na ⁺ or K ⁺ . When it is a zwitterion, it is associated with both positive and negative counterions, such as ^Cl- and Na ⁺ or K ⁺ .

Viscoelastic surfactant of the present invention can have following general formula:

R-Z

where R is the hydrophobic tail of the surfactant, which may be a fully or partially saturated straight or branched hydrocarbon chain having at least 18 carbon atoms, and z is the head group of the surfactant, which may be _-NR R ₂ R ₃ ⁺ , -SO ₃ ^- , -COO ^- , or when the surfactant is a zwitterion, it is -N ⁺ (R ₁ )(R ₂ )R ₃ -COO ^- , where R ₁ , R ₂ and _R3 are each independently hydrogen or a fully or partially saturated straight or branched aliphatic chain having at least one carbon atom; and _R1 or _R2 may contain a hydroxyl end group.

In another example, it can be a splittable viscoelastic surfactant having the following general formula, which is disclosed in the GB 0103449.5 patent application filed on February 13, 2001, which is on the priority date of the present application Not public yet:

R-X-Y-Z

Wherein R is the hydrophobic tail of the surfactant, which can be a fully or partially saturated straight or branched hydrocarbon chain with at least 18 carbon atoms, and X is a cleavable or degradable group of the surfactant, which It can be an acetal, amide, ether or ester bond, Y is a spacer group, which consists of a saturated or partially saturated short hydrocarbon chain containing n carbon atoms, where n is at least equal to 1, preferably equal to 2, when n≥3 When , it can be a linear or branched alkyl chain, and z is the head group of a surfactant, which can be -NR ₁ R ₂ R ₃ ⁺ , -SO ₃ ^- , -COO ^- , or when the surface active When the agent is a zwitterion, it is -N ⁺ (R ₁ R ₂ R ₃ -COO ^- ), wherein R ₁ , R ₂ and R ₃ are each independently hydrogen or a fully or partially saturated straight chain or branched aliphatic chain and may contain a hydroxyl end group. Due to the presence of cleavable or degradable groups, cleavable surfactants are able to degrade under downhole conditions.

The cationic viscoelastic surfactant that is suitable for implementing the present invention is N-caucetyl-N, N-two (2-hydroxyethyl)-N-methyl ammonium chloride. In containing 4wt%NaCl or 3wt%KCl In an aqueous solution of , this viscoelastic surfactant forms a gel containing worm-like micelles, and the worm-like micelles are entangled at a concentration of 1.5 to 4.5 wt%. When the gel is broken up by a hydrocarbon , these worm-like micelles were degraded into spherical micelles.

Anionic viscoelastic surfactants suitable for the practice of the present invention are monocarboxylate RCOO ^- such as oleate, wherein R is C ₁₇ H ₃₃ or di- or oligomeric carboxylate, as in PCT/ Substances disclosed in GB01/03131 patent application, which had not been published on the priority date of the present application. These mono-, di- or oligomeric carboxylates form viscoelastic gels when in alkaline solution in the presence of added salts such as potassium chloride (KCl) or sodium chloride (NaCl). When the gel is broken up by hydrocarbons, the wormlike micelles of the gel degrade into spherical micelles.

Zwitterionic surfactants suitable for the practice of the present invention may be betaine surfactants of the general formula RN(R ₁ R ₂ )-Z, wherein Z is an alkyl group, or betaine surfactants of the general formula R-CN(R ₁ R ₂ R ₃ )-Z betaine surfactant, wherein Z is an acyl group. The hydrophobic group R can be aliphatic or aromatic, linear or branched, saturated or unsaturated. The anionic group Z of the surfactant can be -R'-SO ₃ ^- , -R'-COO ^- , wherein R' is a saturated aliphatic chain. R ₁ , R ₂ and R ₃ are each independently hydrogen or an aliphatic chain having at least one carbon atom.

Hydrophobically modified polymers are soluble in water. It has an average molecular weight of from 10,000 to 10,000,000 g/mol, preferably from about 100,000 to about 2,000,000 g/mol. When the average molecular weight is above 2,000,000, more precisely above 10,000,000 g/mol, the polymer may form structures which are difficult to remove from the fracture during the subsequent flow-back of formation fluids. When the average molecular weight is below 100,000, more precisely below 10,000 g/mol, the polymer concentration necessary to obtain the fluids of the invention is likely to be too high, thereby significantly increasing the costs associated with the fluids.

The hydrophobically modified polymer has a main backbone, and randomly or non-randomly grafted hydrophobic side chains onto the main backbone, and the degree of substitution of the side chains is 0.01 to 10 wt%, and preferably about 0.03 to about 5 wt%. The polymer can be charged or uncharged, the charge can be positive or negative, and the charge can be located on the polymer backbone or on the hydrophobic side chains. If the degree of hydrophobic group substitution of the hydrophobically modified polymer is too high, its solubility in water is reduced. If it is too low, it is difficult to obtain a stable fluid with sufficient viscosity. In fact, by adjusting the degree of substitution of the hydrophobically modified polymer, a satisfactory fluid viscosity with sufficient polymer water solubility is obtained.

The polymeric backbone may be of biological origin. In particular, it may be a polysaccharide. Suitable polysaccharides for the practice of the present invention are starch or starch derivatives, such as phosphate starch, succinic acid starch, aminoalkyl starch or carboxypropyl starch; cellulose or cellulose derivatives, such as carboxymethylcellulose, methylcellulose cellulose, ethylcellulose or hydroxypropylmethylcellulose; chitin or chitin derivatives such as polyglucosamine or polyglucosamine derivatives such as N-carboxybutylglucosamine or N-carboxy Methylpolyglucosamine; galactomannans, in particular guar gum or guar gum derivatives, such as carboxymethyl guar gum or carboxymethylhydroxypropyl guar gum derivatives. It may also be a synthetic polymer such as a polyanhydride, for example poly(isobutylene-alternate-maleic anhydride), poly(ethylene-alternate-maleic anhydride), poly(ethylene-graft-maleic anhydride), polyacrylamide, Polyacrylate, polyacrylate/polyacrylamide copolymer, polyether, polyester, polyamide or polyvinyl alcohol.

The hydrophobic side chain is preferably a fully or partially saturated straight or branched hydrocarbon chain, and said hydrocarbon chain preferably contains from about 12 to 24 carbon atoms, and advantageously includes a cleavable or degradable group such as an acetal , amide, ether or ester linkages.

An example of an uncharged hydrophobically modified polymer which is convenient for the practice of this invention is guar gum which has been hydrophobically modified with uncharged alkyl chains.

An example of a positively charged hydrophobically modified polymer that is convenient for the practice of the present invention is hydrophobically modified polyglucosamine, the charge of which is located on the polymer backbone. This polymer can be described according to Yalpani, M . and Hall, L.D.'s "Macromolecules" (Macromolecules), 1984, vol.17, the route described in p.272 is synthesized at different degrees of hydrophobic substitution, which is shown in Figure 15 according to the free polyglucosamine Reductive amination of amino groups to produce N-alkylated polyglucosamine, or as per D. Plusquellec and al., ENSCR, Department de chimie Organique in "Synthesis of N-glycosylamino acids and N-glycosylamino acids for membrane studies". Efficient Acylation of Free Glycosylamines for the Synthesis of N-GlycosylAmino Acids and N-Glycosidic Surfactants for Membranes Studies of Glycoside Surfactants", J.Carbohydrate Chemistry, 1994, 13(5), 737-751, in this case yielding N-acetylated polyglucosamine with cleavable hydrophobic chains.

Other examples of hydrophobically modified polymers suitable for carrying out the invention are hydrophobically modified polyanhydrides obtainable by amidation or esterification of polyanhydrides, such as poly(isobutylene-alternating-maleic anhydride) , poly(ethylene-alternate-maleic anhydride), or poly(ethylene-graft-maleic anhydride), each bearing an amine or alcohol chain comprising about 12 to about 24 carbon atoms. These hydrophobically modified polyanhydrides include carboxyl groups attached to their backbone, each carboxyl group attached to a hydrophobic side chain. As a result, hydrophobically modified polyanhydrides are not only hydrophobic but also hydrophilic. The chemical structure of the hydrophobic side chain preferably corresponds to, and more preferably matches, the hydrophobic tail of the surfactant molecule of the fluid. In this case, the overall chemical structure of the hydrophobic side chains and the carboxyl groups attached to them, which resemble the charged Hydro head.

Figure 13 depicts poly(ethylene-alternate-maleic anhydride) hydrophobically modified with oleyl side chains, and a synthetic route for such hydrophobically modified polymers. As shown in the figure, the hydrophobically modified poly(ethylene-alt-maleic anhydride) includes a carboxylic acid group ^-COO- attached to the carbon atom immediately adjacent to the carbon atom of the grafted hydrophobic oleyl side chain. In this way, the hydrophilic and hydrophobic structure of the viscoelastic surfactant matches the local structure of the hydrophilic and hydrophobic groups on the polymer. Additionally, the oleyl side chain includes an amide linkage, which is cleavable or degradable.

In addition to surfactants and hydrophobically modified polymers, the fluids of the present invention may also include salts including inorganic salts such as ammonium chloride, chloride sodium chloride and potassium chloride or organic salts such as sodium salicylate. The fluid may also contain organic solvents such as isopropanol, which may be used to liquefy the viscoelastic surfactant component. The fluid may also contain certain fluid loss additives such as a mixture of starch and mica to reduce fluid loss.

The fluids of the present invention are viscoelastic. For example, the viscoelasticity of a fluid can be measured by performing dynamic oscillatory rheological measurements on the composition, as described in Barnes H.A. et al., "An Introduction to Rheology", Elsevier, Amsterdam (1997) generally described in . In a typical dynamic oscillation experiment, the composition is subjected to sinusoidal shear according to the following formula (1):

γ(t)=γ _(max) sinωt (1)

where γ(t) is the strain, γ(max) is the maximum strain, t is the time, and ω is the angular frequency. The shear stress σ is given by:

σ(t)=σ _(max) sin(ωt+δ) (2)

where δ is the phase angle.

The relative inputs given by the elastic modulus (G') and viscous modulus (G") are determined as follows. The sine function extending equation (2) gives the following equations (3) and (4)

σ(t)=σ _(max) [sinωt cosδ+cosωt sinδ] (3)

σ(t)=γ _(max) [G′sinωt+G″cosωt] (4)

Among them, G′≡(σ _(max) /γ _(max) ) cosδ, G″≡(σ _(max) /γ _(max) ) sinδ.

Formula (4) thus defines two dynamic moduli for compositions with viscoelastic properties: G', the storage modulus or elastic modulus; G", the loss modulus or viscous modulus.

The fluid of the present invention is a water-based viscoelastic gel, and the term "viscoelastic gel" as used herein refers to a composition whose elastic modulus (G') is at least as important as its viscous modulus (G") .During the evolution from a predominantly viscous liquid to a viscoelastic gel, the gel point can be defined by the point in time when the contribution of the elastic modulus is equal to that of the viscous modulus, that is, G′=G″ at this time; At and beyond this point in time, G'≥G" and the phase angle δ≥45°.

The viscoelastic properties of the fluids of the present invention are due to the interaction between the hydrophobically modified polymer and "long" (ie worm, thread or rod) micelles of the surfactant. Hydrophobically modified polymers, and especially the hydrophobic side chains of the polymers, interact with viscoelastic surfactants to form long micelles. This interaction is schematically shown in FIG. 1 and is a physical hydrophobic-hydrophobic interaction. The result is an overlapping network.

Micelles form in water at very low surfactant concentrations; the critical micelle concentration (c.m.c.) is the concentration at which micelles (actually spherical micelles) begin to form. c.m.c. is usually measured by surface tension, solubility, conductivity in the case of ionic surfactants, self-diffusion or NMR.

In order to form an overlapping network, the concentration of viscoelastic surfactant should be above its critical micelle concentration (c.m.c.).

Plotting the logarithm of the zero shear viscosity of a water-based fluid containing the viscoelastic surfactant as a function of the logarithm of its concentration yields the overlay concentration c ^* of the viscoelastic surfactant. A non-linear relationship is obtained and c ^* is the concentration of viscoelastic surfactant corresponding to the inflection point between the two linear slopes of the curve (Figure 2).

The concentration of viscoelastic surfactant is advantageously below 10 wt%, preferably below 5 wt%, and below 20 x c ^* , where c ^* is the overlapping concentration of said viscoelastic surfactant. More preferably, it is comprised between 0.2c ^* and 5×c ^* . This corresponds to a concentration of viscoelastic surfactant that is much lower than the concentration of viscoelastic surfactant used in prior art viscoelastic surfactant fracturing fluids, which is on the order of 30-40×c ^* .

A plot of the logarithm of the zero-shear viscosity of a hydrophobically modified polymer fluid as a function of the logarithm of its concentration has two critical concentrations: (1) the overlap concentration c ^* and (2) the entanglement concentration c _e . A curve is obtained, wherein c ^* and c _e are the concentration of the hydrophobically modified polymer corresponding to the two inflection points of the three slopes of the curve. The dilute region of hydrophobically modified polymer concentration is defined as c<c ^* . At this concentration c, the zero shear viscosity is of the order of the solvent viscosity. The semi-dilute unentangled region of the hydrophobically modified polymer concentration is defined as strictly contained between c ^* and _ce . At this concentration c, the viscoelasticity of the fluid is governed by Rouse kinetics and its viscosity increases moderately. The semi-lean entanglement region for hydrophobically modified polymer concentrations is defined as c > c _e . At this concentration c, the viscosity of the fluid can be described by the reptation model.

In the overlapping network of the fluid of the present invention, the concentration of the hydrophobically modified polymer is lower than its entanglement concentration and higher than its overlapping concentration c ^* .

和R.Zana的“季胺盐表面活性剂低聚物与改性的水溶性瓜耳胶之间的相互作用(Interactionsbetween quaternary ammonium surfactant oligomers andwater-soluble modified guars)”，《胶体和界面科学期刊》(Journalof Colloid and Interface Science)，1999，218：468-479；S.Biggs，J.Selb和F.Candau的“表面活性剂对疏水改性的聚丙烯酰胺的溶液特性的影响(Effect of surfactant on the solution properties ofhydrophobically modified polyacrylamide)”，Langmuir，1992，838-847；A.Hill，F.Candau和J.Selb的“疏水相连的共聚物的水溶液特性(Aqueous solution properties of hydrophobicallyassociating copolymers)”，《胶体和聚合物科学的进展》(Progressin Colloid & polymer Science)，1991，84：61-65；O.Anthony，C.M.Marques和P.Richetti的“阳离子瓜耳胶在带相反电荷的表面活性剂的溶液中的主体和表面行为(Bulk and surface behavior of Cationicguars in solutions of oppositely charged surfactants)”，Langmuir，1998，14：6086-6095；I.Iliopoulos的“疏水聚合电解质和表面活性剂之间的结合(Association between hydrophobicpolyelectrolytes and surfactants)”，《胶体和界面科学的现代观点》(Current Opinion in Colloid & Interface Science)，1998，3：493-498；S.Panmai、R.K.Prud′homme和D.Peiffer的“含有球状和棒状表面活性剂胶束的疏水改性的聚合物的流变特性(Rheology ofhydrophobically modified polymers with spherical and rod-likesurfactant micelles)”，化学工程系(Department of ChemicalEngineering)，Princeton University，Princeton，NJ、Exxon Researchand Engineering Company，Annondale，NJ，1997；以及公开专利US-4,975,482、US-5,036,136和US-6,194,356。在某些情况下，这些研究的教导对于理解本发明的流体中存在的相互作用是很有用的。Certain interactions between polymers and surfactant molecules have been studied and corresponding results can be found in: "Associative polymers in aqueous solution" by MA Winnik and A. Yekta ", "Current Opinion in Colloid & Interface Science", 1997, 2: 424-436; U.

and R. Zana, "Interactions between quaternary ammonium surfactant oligomers and water-soluble modified guars", Journal of Colloid and Interface Science (Journalof Colloid and Interface Science), 1999, 218: 468-479; S.Biggs, J.Selb and F.Candau's "surfactant to hydrophobically modified polyacrylamide solution characteristics (Effect of surfactant on the solution properties ofhydrophobically modified polyacrylamide), Langmuir, 1992, 838-847; A.Hill, F.Candau and J.Selb's "Aqueous solution properties of hydrophobically associated copolymers (Aqueous solution properties of hydrophobically associating copolymers)", "Colloid "Progressin Colloid & polymer Science", 1991, 84:61-65; O.Anthony, CM Marques and P.Richetti's "Cation of Guar Gum in a Solution of Oppositely Charged Surfactant Body and surface behavior (Bulk and surface behavior of Cationicguars in solutions of oppositely charged surfactants)", Langmuir, 1998, 14: 6086-6095; I. Iliopoulos "Association between hydrophobic polyelectrolytes and surfactants and surfactants), "Current Opinion in Colloid & Interface Science", 1998, 3: 493-498; S. Panmai, RK Prud′homme and D. Peiffer "Containing spherical and rod-like surfaces Rheology of hydrophobically modified polymers of active agent micelles polymers with spherical and rod-likesurfactant micelles), Department of Chemical Engineering (Department of Chemical Engineering), Princeton University, Princeton, NJ, Exxon Research and Engineering Company, Annondale, NJ, 1997; and published patents US-4,975,482, US-5,036,136 and US -6,194,356. In some cases, teachings from these studies were useful in understanding the interactions that exist in the fluids of the present invention.

The fluids of the present invention are hydrocarbon responsive such that the gel structure ruptures upon contact with or mixing with hydrocarbons. The long viscoelastic surfactant micelles that together form the gel network with the hydrophobically modified polymer degrade on contact with hydrocarbons to form spherical micelles. Concentrations of hydrophobically modified polymers below c _e are insufficient to form entangled networks. Then, at high shear rates, the viscosity of the gel is reduced to about 100 cP or less, preferably 20 cP.

The leakage rates of the fluids of the present invention are preferably lower than those of pure viscoelastic surfactant fluids of equivalent rheological properties. This is a very significant advantage. As a result, the responsive fluids of the present invention can be used to fracture higher permeability formations than pure viscoelastic surfactant fluids. Relative to pure viscoelastic surfactant fluids, it is likely that after gel degradation through interaction with hydrocarbons, the polymer component may prevent the cleaning of the crevice. But it is also worth noting that the cleaning performance of the fluids of the present invention may be similar to or better than that observed for low concentration linear polymer fracturing fluids, i.e. the cleaning performance should be acceptable and better than that of higher concentration linear polymer fracturing fluids. Polymer fluids or covalently cross-linked polymer fluids.

Virtually all compounds of the fluids of the present invention are mixed at the surface with proppant, which may be 20-40 mesh sand, alumina or glass beads. When processed at very high shear rates, the viscosity of this fluid is low enough to allow it to be pumped downhole. The proppant-laden pumped fluid is then injected at high pressure into the formation rock to be fractured. At this time, the fluids of the present invention are sufficiently viscous to carry proppant through the fracture. The fluid is then degraded by contact with hydrocarbons flowing through the fracture.

Example 1

Determination of the overlapping concentration c ^* of viscoelastic surfactants at different temperatures

Figure 2 as a function of concentration, plotted at 25, 40 and 60 ° C N-caucetyl-N, N-bis (2-hydroxyethyl) -N-methyl ammonium chloride at 3 wt% Zero Shear Viscosity in Potassium in Aqueous Solution, where said concentration is calculated as wt % of N-Cericyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride.

Three curves are obtained. From these curves, it can be deduced that the critical overlap concentration c ^* of the viscoelastic surfactant is equal to 0.08 wt% at 25°C, 0.1 wt% at 40°C and 0.15 wt% at 60°C. It is reasonable for c ^* to increase with increasing temperature, since the average length of wormhole micelles is expected to decrease with increasing temperature.

Example 2

_Determining the overlap concentration c ^* and entanglement concentration c* of hydrophobically modified polymers at different temperatures

Figure 3 plots the zero-shear viscosity of hydrophobically modified hydroxypropyl guar (hm-HPG) in 3 wt % potassium chloride in water at 25, 40, 60 and 80 °C as a function of concentration, Wherein said concentration is calculated by weight % of said hydrophobically modified polymer. Hydrophobically modified hydroxypropyl guar gum has a molecular weight of about 0.5 x ¹⁰⁶ g/mol and contains about 0.03 to 1.7 wt% of linear hydrocarbon side chains having 22 carbon atoms.

Four curves are obtained. From these curves the critical overlap concentration c ^* and entanglement concentration c _e of hydrophobically modified hydroxypropyl guar can be deduced. c ^* corresponds to the inflection points of the first two slopes of each curve, which are equal to 0.1 ± 0.05 wt% at 25, 40, 60 and 80°C. c _e correspond to the inflection points of the second and third slopes of each curve, which are equal to 0.35±0.05 wt% at 25, 40, 60 and 80°C. The term "about" for c ^* or _ce is generally considered to mean c ^* ± 0.05 wt% or c _e ± 0.1 wt%.

Example 3

Comparison of rheological properties of pure viscoelastic surfactant fluids with that of the same viscoelastic surfactant fluids containing different concentrations of hydrophobically modified polymers

Figure 4 as a function of concentration, plotting the concentration of N-caulenyl-N,N at 80°C at a high shear rate of 100 ^s - the viscosity of a water-based solution of hydrophobically modified hydroxypropyl guar gum mixed with a 3 wt% potassium chloride solution of bis(2-hydroxyethyl)-N-methylammonium chloride, wherein said concentration is expressed in said Calculated as wt% of N-caucetyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride. The hydrophobically modified hydroxypropyl guar gum has a molecular weight of about 0.5×10 ⁶ g/mol and includes 0.03 to 1.7 wt % of linear hydrocarbon side chains having 22 carbon atoms.

Figure 4 is a graph depicting the addition of a small amount of hydrophobically modified hydroxypropylguaranine to a water-based solution of N-callenyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride. After ear gel (hm-HPG), the viscosity of the mixed fluid was the same as that of the pure N-callenyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride considered at the same concentration The viscosity of the fluid is much higher in comparison.

To obtain a fluid with a viscosity of 100 cP at high shear rate (100 s ^-1 ) requires 2.3 wt% of N-Cericyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride . By adding 0.4 wt% hm-HPG to a 0.3 wt% viscoelastic surfactant aqueous solution, the concentration of viscoelastic surfactant can be reduced by a factor of 7.5. Taking into account the individual cost of each component, the cost of the blended fluid is about 4 times less than that of the pure viscoelastic surfactant fluid.

Example 4

Fluid responsiveness to hydrocarbons

The responsiveness of fluids to hydrocarbons is tested using the bottle test. The experiment was carried out on three different fluids in the presence of oil (mineral spirits of the boiling fraction between 179°C and 210°C). A volume of each fluid was placed in the bottle and an equal volume of oil was placed on top of the gel. Each bottle was sealed and heated in an oven at 60°C for 1 hour. Each bottle was then visually inspected to determine if the gel had been broken down to its base viscosity. If not already broken, the gel sample was shaken vigorously for 20 seconds and the bottle was placed in the oven for an additional 1 hour. This process was repeated until the gel was broken up and the total time taken for this process was recorded. Once the gel has broken up, heat the bottle and shake for a further 3-6 hours to determine if any emulsification has occurred.

The first fluid is a water-based liquid containing 0.7 wt% hydrophobically modified hydroxypropyl guar gum and 3 wt% KCl. The hydrophobically modified hydroxypropyl guar gum has a molecular weight of about 0.5×10 ⁶ g/mol, And contain 0.03 to 1.7 wt% of linear hydrocarbon side chains having 22 carbon atoms. The second fluid is a water-based fluid composed of 3wt% N-caucetyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and 3wt% KCl, while the third The fluid is hydrophobically modified hydroxypropyl guar gum of 0.7wt%, 3wt% of N-callenyl-N, N-di(2-hydroxyethyl)-N-methylammonium chloride and 3wt% A water-based fluid composed of %KCl.

By contact with the oil, after 2 to 3 hours all the gel in each bottle had been broken up and we could see two clear phases, the lower phase containing the broken gel and the upper phase containing the oil. But in the bottle containing the third fluid shown on the left side of Fig. 5, the oil phase is clear and only a little emulsion is observed, while in the other two bottles, the oil phase appears due to the presence of the emulsion phase A little cloudy. After an extended period of time, an emulsion forms in the third fluid.

Thus, water-based fluids containing surfactants and hydrophobically modified polymers are responsive to hydrocarbons and delay emulsion formation after breakdown.

In Fig. 6 plotted at different hm-HPG concentrations 0.1, 0.4 and 0.8wt%, and at different viscoelastic surfactant concentrations 1, 2 and 3wt%, the hm-HPG/viscoelastic surfactant fluid The relationship between viscosity and break time. Mixtures containing hm-HPG at concentrations equal to or lower than c ^* collapse to very low viscosities similar to those of pure viscoelastic surfactant fluids of equivalent initial viscosity. However, those with higher polymer concentrations (especially when c> _ce ) exhibit relatively higher viscosities after rupture, equal to that of the corresponding non-hydrophobically modified hydroxypropyl guar (HPG) viscosity.

Example 5

Requirements for Hydrophobic Side Chains

In the first bottle shown on the left side of Figure 7, the following first mixture was charged: 0.7 wt% of the hydrophobically modified hydroxypropyl guar gum of Example 2, 1 wt% of the viscoelastic surface of Example 1 active agent and 3 wt% potassium chloride. In the second bottle shown on the right side of Figure 7, the following second mixture was charged: 0.7 wt% of the corresponding non-hydrophobically modified hydroxypropyl guar gum, 1 wt% of the viscoelastic Surfactant and 3 wt% KCl.

In the first bottle only one phase could be distinguished, while in the second bottle there were two phases present. Therefore, no phase separation occurs in the first bottle because the hydrophobic association between the hydrophobically modified polymer and the viscoelastic surfactant stabilizes the mixture, whereas in the second bottle there is phase separation because the unmodified There is no stable interaction between the polymer and the viscoelastic surfactant.

Example 6

Comparison of Fluids Containing Anionic and Cationic Surfactants

Figure 8 plots the viscosity of a water-based fluid containing 0.6 ^wt % of the hydrophobically modified hydroxypropyl Base guar gum and cationic surfactant N-dimeryl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride or anionic surfactant dimer acid potassium chloride C ₃₆ H ₆₈ O ₄ K ₂ .

The viscosity of fluids containing anionic surfactants is not as effective as that obtained with fluids containing cationic surfactants. This is believed to be due to differences in the structure of the surfactant aggregates rather than differences in charge. Cationic surfactants can form wormlike micelles at 80°C, but anionic dimers cannot. Mixtures containing the same hydrophobically modified guar gum but also containing an anionic surfactant which formed wormlike micelles at 80°C were then evaluated. The anionic surfactant is oleylamide succinate C ₂₂ H ₄₀ NO ₃ ⁻ Na ⁺ . The data obtained are shown in Figure 9. Now it can be observed that the hydrophobically modified polymer/cationic surfactant and the hydrophobically modified The polymer/anionic surfactant mixtures have similar viscosities. This result confirms that it is the structure of the surfactant aggregates rather than the charge of the surfactant that affects its interaction with the hydrophobically modified polymer.

In addition, the same test as in Example 4 was carried out using the fluid described above. The results indicated that these fluids were responsive to hydrocarbons and indicated that emulsion formation was delayed.

Example 7

Viscosity properties of mixtures as a function of temperature compared to pure polymer and pure VES fluid

In Figure 10, the viscosity of the following fluids at a high shear rate of 100 s ^-1 is plotted as a function of temperature: - with 0.4 wt% of the hydrophobically modified hydroxypropyl guar of Example 2 and 3 wt% of KCl The water-based fluid of - containing 2wt% the cationic surfactant N-callenyl-N, the water-based fluid of N-bis(2-hydroxyethyl)-N-methylammonium chloride and 3wt% KCl, - Containing 0.4 wt% of the hydrophobically modified hydroxypropyl guar gum of Example 2 and 2 wt% of N-caucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and 3wt% potassium chloride water-based fluid, or the hydrophobically modified hydroxypropyl guar gum of 0.7wt% embodiment 2 and 2wt% N-caulidyl-N, N-bis(2-hydroxy Aqueous based fluid of ethyl)-N-methylammonium chloride and 3 wt% potassium chloride.

The results show that the viscosities of pure viscoelastic surfactant fluids and pure polymer fluids decrease when the temperature rises above 80 °C, while the viscosity of the hm-polymer/viscoelastic surfactant system remains high (at high It is still higher than 50cP at about 110°C). The viscosity still increases when increasing the concentration of hydrophobically modified polymer.

Example 8

Changes in fluid viscosity as a function of different parameters such as temperature, salt concentration

In Figure 11, the viscosity of the following water-based solutions at 140°C at a high shear rate of 100 ^s is plotted as a function of concentration: 0.4 wt% of the hydrophobically modified hydroxypropyl guar of Example 2 Mixtures of gum and betaine _C29H56N2 ⁺ _O3- in potassium chloride at different _{concentrations} (0, ₁ and 4 wt %), where said concentrations are calculated as wt% of said ^betaine . Viscosity was increased from 20 cP to 76 cP by adding 0.4 wt % hm-HPG to 1 wt % betaine fluid in 4 wt % KCL. Also, the salt concentration has no real effect on the viscosity of the mixture.

In Figure 12, the viscosities of the following water-based fluids at high shear rates of 100 s ^-1 are plotted as a function of temperature: - containing 0.4 wt% hydrophobically modified hydroxypropyl guar and containing 4 wt% chlorine 0.5 wt% betaine water-based fluid containing potassium chloride,-containing 0.4 wt% hydrophobically modified hydroxypropyl guar gum and 1 wt% betaine containing 4 wt% potassium chloride,-or containing A water-based fluid of 4 wt% potassium chloride in 2 wt% betaine.

In contrast to pure betaine, a fluid containing 4 wt% KCl in 2 wt% betaine had a similar viscosity to a fluid containing 0.4 wt% hm-HPG and 0.5 wt% betaine and 4 wt% KCl. This conclusion is true up to 100°C, beyond which and up to 140°C, the viscosity of a fluid containing 4wt% KCl in 2wt% betaine is comparable to that of a fluid containing 0.4wt% hm-HPG and 1wt% betaine and 4wt% The fluid viscosity of KCl is comparable.

Example 9

Changes in fluid viscosity as a function of different parameters such as temperature

Following the route shown in Figure 13, hydrophobically modified polyanhydrides with molecular weights ranging from 100,000 to 500,000 g/mol were synthesized from the base polymer poly(ethylene-alternating-maleic anhydride). From 1 to 5% by weight of the anhydride units of the hydrophobically modified polyanhydride are converted to oleylamide carboxylate. These hydrophobically modified polyanhydrides are mixed with oleylamide succinate and a cleavable surfactant having the same hydrophobic/hydrophilic side chains as the hydrophobically modified polyanhydrides. structure. At a concentration of 3 wt%, oleylamide succinate surfactant forms rod-like micelles with 4 to 12 wt% KCl or NaCl.

As the degree of hydrophobic substitution of the hydrophobically modified polyanhydrides increases from 1% to 5%, the results show that the overlap concentration c ^* of pure hydrophobically modified polyanhydrides decreases.

In addition, the results show that the overlap concentration c ^* of pure hydrophobically modified polyanhydrides decreases when the salt NaCl or KCl is added. Increasing the ionic strength of hydrophobically modified polymer solutions is expected to reduce the repulsion between charged sites, but also exhibits an enhancement of hydrophobic-hydrophobic interactions.

At 80°C and a high shear rate of 100 s ^-1 , the following viscosities were obtained for the following water-based fluids, where the viscoelastic surfactant (VES) was oleylamide succinate, hydrophobically modified polymer (hm-P ) is the hydrophobically modified polyanhydride substituted by 2.5 wt% in Figure 13:

Water-based fluid composition Viscosity (Cp) of the fluid at a shear rate of 100s^-1 4wt% VES+8wt% KCl 300 3wt% hm-P+8wt% NaCl 11.9 3wt% hm-P+0.6wt% VES+8wt% NaCl 805

The results show that the viscosity of fluids containing both oleylamide succinate and hydrophobically modified polyanhydride is very high, even at very low concentrations of surfactant.

Figure 14 compares the rheological diagrams of the following water-based fluids at a high shear rate of 100 s ^-1 at a temperature of 60-150°C: containing 3 wt% of the hydrophobically modified polyanhydride of Figure 11 with a degree of substitution equal to 2.5, containing 8 wt% NaCl and 0, 0.2, 0.3 or 0.6 wt% oleylamide succinate.

The viscosities of the fluids containing 0.2, 0.3 or 0.6 wt% oleylamide succinate were significantly higher than those without the oleylamide succinate.

Example 10

Change in zero-shear viscosity of different fluids containing hydrophobically modified polyglucosamine and/or viscoelastic surfactants as a function of the concentration of hydrophobically modified polyglucosamine

Hydrophobically modified polyglucosamine with a molecular weight of about 100,000-500,000 g/mol was synthesized from the base polymer polyglucosamine according to the route shown in FIG. 15 . The degree of substitution of linear hydrocarbon side chains with 11 carbon atoms varies between 1 wt% and 7.5 wt%.

Plotted in Figures 16a and 16b at 25°C (Figure 16a) and 60°C (Figure 16b) as a function of the concentration of hm5-polyglucosamine and maintaining the ratio between polymer/viscoelastic surfactant When equal to 1, the logarithm of the zero-shear viscosity of the following water-based fluids: - containing hydrophobically modified polyglucosamine (hm5-polyglucosamine) with 5 mol% hydrophobic substituents and 3 wt% potassium chloride Water-based solution, - a water-based solution of the cationic surfactant N-caucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride, and hm5-polyglucosamine/N - Aqueous-based solution of a causticyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride mixture.

Figures 16a and 16b show that by adding a small amount of N-scolecyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride to an aqueous-based solution of hm5-polyglucosamine, and Compared to pure N-caulidyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride fluid at the same concentration, or compared to pure hm5-polyglucosamine fluid at the same concentration , the viscosity of the mixture will be much higher.

Example 11

Effect of Core Saturation (Oil or Saline Saturation) State on Static Leakage of Hydrophobically Modified Polymer/Viscoelastic Surfactant or Viscoelastic Surfactant Fluids

Under a constant pressure gradient (1000 psi) and temperature (60°C), using a static seepage device and a brine-saturated sandstone core (brine permeability 7-9 mD) with a length of 1 inch and a diameter of 1 inch, the Hydrophobic modified hydroxypropyl guar gum of example 2 and N-caucetyl-N, N-bis(2-hydroxyethyl)-N-methylammonium chloride fluid leakage performance and pure viscous compared to elastic surfactant systems. From Figure 17, it can be observed that the data for a pure viscoelastic surfactant system containing 1 wt% viscoelastic surfactant and 3 wt% KCl is characterized by high leakage in the early stages, i.e., in the time period from 0 to 10 min The rate was an average of 3.9 mL/min, which then decreased to an average of about 0.27 mL/min over a period of 10 to 40 min. The leakage volume in 60 minutes is about 50mL. This pure viscoelastic surfactant system does not form an external filter cake. Addition of 0.2wt% hm-HPG to 1wt% viscoelastic surfactant fluid resulted in a decrease in the early leakage rate, but the resulting total volume increased from about 50 mL to 80 mL after 60 min. Unlike pure viscoelastic surfactants, an outer polymer cake is now formed by mixing. Only when the added dose of hm-HPG was increased to 0.5 wt%, the total volume collected decreased from about 50 mL to about 42 mL within a period of 60 min.

The results show that the initial saturation state of the core, ie brine saturation or oil partial saturation, has a large influence on the seepage behavior. At 60°C and a constant pressure of 1000psi, when the initial saturation state changes from water saturation Sw = 1 to oil saturation So = 0.8, that is, when the irrecoverable water saturation Sw = 0.2 (Figure 18), 1 wt% viscoelastic surface activity The 1-hour seepage volume of the agent concentrate was increased from 50 mL to 190 mL. As the gel enters the core, its structure and viscosity are broken by the oil, resulting in higher flow rates. At a later time period, when the oil had been removed from the core, the flow rate decreased to a level similar to that observed in the brine saturation situation. In contrast, under the same conditions, the 1-hour leakage volume of the mixture of 1 wt% viscoelastic surfactant and 0.2 wt% hm-HPG is equal to 80 mL, and under the same change of initial saturation state (Figure 19), the The leakage volume remains unaffected. This suggests that when hydrophobically modified polymers are present, the percolation rate is largely controlled by the external filter cake.

Example 12

Effect of Hydrophobically Modified Polymer Concentration on Leakage Rate of Hydrophobically Modified Polymer/Viscoelastic Surfactant Fluids

Under constant pressure gradient (1000psi) and temperature (80°C), applying static seepage equipment and oil-saturated sandstone cores (brine permeability 7-9mD) 1 inch in length and 1 inch in diameter, comparatively containing 2wt Hydrophobic properties of % N-caucetyl-N, N-bis(2-hydroxyethyl)-N-methylammonium chloride, 3wt% KCl and different concentrations (0.1, 0.2 and 0.4wt%) of Example 2 Fluid leakage properties of modified hydroxypropyl guar gum.

As shown in Fig. 20, the results show that when hm-polymer was added to 2wt% viscoelastic fluid, the leakage rate decreased significantly. This reduction depends on the concentration of hydrophobically modified polymer. By adding 0.2wt% hm-HPG, the leakage rate of 2wt% viscoelastic fluid was reduced by a factor of 2 after 0.5-1 hour. This reduction was even greater as the concentration of hm-HPG added was increased.

Example 13

Comparison of the leakage performance of hydrophobically modified polymer/viscoelastic surfactant fluids and pure viscoelastic surfactant fluids of the same viscosity at different saline permeability

Under a constant pressure gradient of 1000 psi and 60 °C, the hydrophobically modified hydroxypropyl guar gum/N-caucetyl-N, N-bis(2-hydroxyethyl)-N-methyl The seepage performance of ammonium chloride fluid was compared to a pure viscoelastic surfactant system using a static seepage device and an oil-saturated sandstone core 1 inch in length and 1 inch in diameter (brine permeability 7 -9mD and 25-35mD).

As shown in Figure 21, the leakage rate of the oil-saturated core was significantly reduced when hm-HPG was added to the viscoelastic surfactant gel at a permeability of 7-9 mD. After 60 minutes, the leakage volume of 3wt% viscoelastic surfactant fluid is equal to 33mL, and the same low shear viscosity as 3wt% viscoelastic surfactant fluid containing 2wt% viscoelastic surfactant and 0.4wt% hm - The leakage volume of the fluid of the HPG is equal to 18 mL. For oil saturated cores, at permeability of 25-35 mD, the hm-HPG/viscoelastic surfactant blend was not effective when the same concentration of polymer was used at low permeability.

Example 14

Comparison of the leakage performance of hydrophobically modified polymer/viscoelastic surfactant fluids and pure viscoelastic surfactant fluids with and without fluid loss additives

Under a constant pressure gradient of 1000 psi and 60 ° C, the permeation of a fluid containing 1 wt % N-caucyl-N, N-di(2-hydroxyethyl)-N-methylammonium chloride and 3 wt % KCl The leak performance was compared to the same fluid to which 0.18 wt% or 0.36 wt% fluid loss additive was added. The fluid loss additive is a mixture of starch and mica. Seepage experiments were performed using a static seepage apparatus and oil-saturated sandstone cores (brine permeability 8-9 mD) 1 inch in length and 1 inch in diameter.

As shown in Figure 22, when a fluid loss additive is added to a viscoelastic surfactant fluid, the leakage rate is significantly reduced. After 60 minutes, the leakage volume of the viscoelastic surfactant fluid was equal to 110 mL, while the leakage volume of the same fluid containing 0.18 wt % fluid loss additive was equal to 36 mL, and when 0.36 wt % fluid loss additive was added, the leakage volume Equal to 16.9mL.

Under a constant pressure gradient of 1000psi and 60°C, the hydrophobically modified hydroxypropyl guar gum containing 0.2wt% of Example 2, 1wt% N-callenyl-N,N-di(2-hydroxyethyl guar gum) Base)-N-methylammonium chloride and 3wt% KCl fluid leakage performance compared with the same fluid to which 0.18wt% or 0.36wt% fluid loss additive was added. The fluid loss additive is a mixture of starch and mica. Seepage experiments were performed using static seepage devices and oil-saturated sandstone cores (brine permeability 8-9 mD).

As shown in Figure 22, when fluid loss additives were added to the mixed fluid, the leakage rate was significantly reduced. After 60 minutes, the leakage volume of the mixed fluid was equal to 85 mL, while the leakage volume of the same fluid with 0.18 wt % fluid loss additive was equal to 17.6 mL, and when 0.36 wt % fluid loss additive was added, the leakage volume was equal to 16.2 mL . Fluid loss additives reduce the permeability of the polymer filter cake, thereby reducing the leakage rate.

For fluid loss additive containing 0.18wt%, after 1 hour, the leakage volume of mixed fluid (1wt% viscoelastic surfactant and 0.2wt% hm-HPG) was 17.6mL, which is about pure viscoelastic surfactant (1 wt% viscoelastic surfactant) is half of the leakage volume of 36 mL.

Claims (32)

1. A water-based viscoelastic fracturing fluid for hydrocarbon recovery comprising a viscoelastic surfactant and a hydrophobically modified polymer, wherein the hydrophobically modified polymer comprises cleavable hydrophobic sides chain, and the concentration of the hydrophobically modified polymer is between its overlap concentration c ^* and entanglement concentration c _e , where as a function of the logarithm of its concentration, the zero-shear viscosity of the hydrophobically modified polymer fluid The logarithmic line of the logarithmic line has two critical concentrations: (1) overlapping concentration c ^* and (2) entanglement concentration c _e , where c ^* and c _e correspond to the two inflection points of the three slopes of the logarithmic line The concentration of the hydrophobically modified polymer. 2. The fracturing fluid of claim 1, wherein the concentration of the viscoelastic surfactant is above its critical micelle concentration. 3. The fracturing fluid of claim 1 or 2, wherein the concentration of viscoelastic surfactant is less than 10 wt%. 4. The fracturing fluid of claim 3, wherein the concentration of viscoelastic surfactant is less than 5 wt%. 5. The fracturing fluid of claim 1 or 2, wherein the concentration of viscoelastic surfactant is less than 20 x c ^* , where c ^* is the overlapping concentration of said viscoelastic surfactant. 6. The fracturing fluid of claim 5, wherein the concentration of viscoelastic surfactant is less than 10 x c ^* . 7. The fracturing fluid of claim 1 or 2, further comprising a salt. 8. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant forms worm, thread or rod micelles. 9. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant is ionic. 10. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant has the general formula: R-Z where R is the hydrophobic tail of the surfactant, which is a fully or partially saturated straight or branched hydrocarbon chain having at least 18 carbon atoms, and Z is the head group of the viscoelastic surfactant, which is -NR ₁ R ₂ R ₃ ⁺ , -SO ₃ ^- , -COO ^- or -N ⁺ (R ₁ )(R ₂ )R ₃ -COO ^- , wherein R ₁ , R ₂ and R ₃ are independently hydrogen or have at least one carbon A fully or partially saturated straight or branched aliphatic chain of atoms, and wherein R ₁ or R ₂ contains or does not contain hydroxyl end groups. 11. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant contains a head group and a hydrophobic tail, and has the general formula: R-X-Y-Z Wherein R is the hydrophobic tail of the viscoelastic surfactant, which is a fully or partially saturated linear or branched hydrocarbon chain with at least 18 carbon atoms; X is a degradable acetal, amide, ether or ester linkage ; Y is a spacer group formed by a short fully or partially saturated hydrocarbon chain having at least one carbon atom; and Z is the head group of the surfactant, which is -NR ₁ R ₂ R ₃ ⁺ , -SO ₃ ^- , -COO ^- or -N+R ₁ R ₂ R ₃ -COO ^- , wherein R ₁ , R ₂ and R ₃ are each independently hydrogen or straight-chain or branched saturated fat with at least one carbon atom family chain. 12. The fracturing fluid of claim 11, wherein Y is formed from short fully or partially saturated hydrocarbon chains having 2 carbon atoms. 13. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant is N-caucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride. 14. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant is a mono-, di- or oligomeric carboxylate. 15. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant is oleylamide _{succinate C22H40NO3} ^- _Na ⁺ . 16. The fracturing fluid of claim 1 or 2, wherein the viscoelastic surfactant is a betaine surfactant having the general formula: RN(R ₁ R ₂ )-Z, wherein Z is an alkyl group, or R-CN(R ₁ R ₂ R ₃ )-Z, wherein Z is an acyl group, And wherein the hydrophobic group R is aliphatic or aromatic, linear or branched, saturated or unsaturated, and the anionic group Z of the surfactant is -R'-SO ₃ ^- , -R' ^-COO- , wherein R′ is a saturated aliphatic chain, and R ₁ , R ₂ and R ₃ are each independently hydrogen or an aliphatic chain having at least one carbon atom. 17. The fracturing fluid of claim 1 or 2, wherein the average molecular weight of the hydrophobically modified polymer is 10,000-10,000,000 g/mol. 18. The fracturing fluid of claim 17, wherein the average molecular weight of the hydrophobically modified polymer is from about 100,000 to about 2,000,000 g/mol. 19. The fracturing fluid of claim 1 or 2, wherein the hydrophobically modified polymer has a main backbone and said hydrophobic side chains are grafted onto said backbone. 20. The fracturing fluid of claim 19, wherein the hydrophobic side chains are grafted onto the backbone with a degree of substitution of 0.01-10. 21. The fracturing fluid of claim 20, wherein the degree of substitution is from about 0.05 to about 5. 22. The fracturing fluid of claim 19 or 20, wherein the polymer backbone is polysaccharide, polyanhydride, polyacrylamide, polyacrylate, polyacrylate copolymer, polyester, polyether, polyamide or polyvinyl alcohol . 23. The fracturing fluid of claim 19, wherein the hydrophobic side chains are fully or partially saturated linear or branched hydrocarbon chains. 24. The fracturing fluid of claim 1 or 2, wherein the hydrophobically modified polymer is hydrophobically modified polyglucosamine. 25. The fracturing fluid of claim 1 or 2, wherein the hydrophobically modified polymer is a hydrophobically modified polyanhydride. 26. The fracturing fluid of claims 1 or 2, wherein the fracturing fluid is capable of forming a hydrocarbon responsive gel. 27. The fracturing fluid of claim 26, wherein the gel is capable of fracturing when contacted or mixed with hydrocarbons. 28. The fracturing fluid of claim 27, wherein the gel is capable of fracturing under downhole conditions to a viscosity of less than 20 cP in less than 2 hours. 29. The fracturing fluid of claim 1 or 2, wherein the fracturing fluid has a leakage rate that is lower than that of a pure viscoelastic surfactant fluid of the same rheology. 30. A method for fracturing formation rock to recover hydrocarbons, the method comprising the steps of: Provided is a water-based viscoelastic fracturing fluid comprising a cleavable viscoelastic surfactant and a hydrophobically modified polymer, wherein the hydrophobically modified polymer includes a cleavable hydrophobic side chain , and the concentration of the hydrophobically modified polymer is between approximately its overlap concentration c ^* and entanglement concentration c _e , where as a function of the logarithm of its concentration, the zero-shear viscosity of the hydrophobically modified polymer fluid The logarithmic line of the logarithmic line has two critical concentrations: (1) overlapping concentration c ^* and (2) entanglement concentration c _e , where c ^* and c _e correspond to the two inflection points of the three slopes of the logarithmic line The concentration of the hydrophobically modified polymer; and The fracturing fluid is injected into formation rock to fracture the rock. 31. The method of claim 30, wherein the concentration of viscoelastic surfactant is above its critical micelle concentration and below 20 x c ^* , where c ^* is the overlapping concentration of said viscoelastic surfactant. 32. The method of claim 30 or 31, wherein the fracturing fluid fractures to a viscosity of less than 20 cP under downhole conditions in less than 2 hours.

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